1. Field of the Invention
The present invention relates to a superconductive high temperature composite thin film material and a method of preparation wherein the composite material has interspersed superconductor and insulator grains. The superconductive composite thin film material has a predetermined critical current density, a high critical temperature and easily moved Josephson vortices. The critical current density is determined by the relative amounts of high temperature superconductor and insulator. The invention particularly relates to a composite film of a superconductor such as YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO), TlCa.sub.2 Ba.sub.2 Cu.sub.3 O.sub.8, Bi.sub.2 (Sr,Ca).sub.3 Cu.sub.2 O.sub.8, La.sub.2-x Sr.sub.x CuO.sub.4 or other rare earth barium copper oxide superconductors and one or more non-diffusing insulators such as cerium oxide, (CeO.sub.2), magnesium oxide (MgO), LaAlO.sub.3 or SrTiO.sub.3 deposited on a substrate that is compatible with the high temperature superconductor and with the superconductive high temperature composite material having easily moved Josephson vortices.
2. Description of the Related Art
There is considerable interest in high transition temperature superconducting devices and high temperature superconducting electronics research focusing on the development of active three-terminal devices having gain. See J. E. Nordman, Semiconductor Science and Technology, 8, 681 (1995); R. Gerdemann, L. Alff, A. Beck. O. M. Froelich, B. Mayer and R. Gross, IEEE Transactions in Applied Superconductivity 5, 3292 (1995); Y. M. Zhang, D. Winkler, P. A. Nilsson, and T. Claeson, Applied Physics Letters, 64 1153 (1994); and K. Miyahara, S. Kubo, and M. Suzuki, Journal of Applied Physics 76, 4772 (1994), all herein incorporated by reference. The vortex flow transistor reported by J. S. Martens. G. K. G. Hohenwarter, J. B. Beyer, J. E. Nordman and D. S. Ginley, J. Appl. Phys. 65, 4057 (1989), herein incorporated by reference, the fluxonic junction transistor proposed by A. M. Kadin, J. Appl. Physics 68, 5741(1990) herein incorporated by reference and the long Josephson junction biased in the flux flow mode reported by D. P. McGinnis, J. E. Nordman and J. B. Beyer, IEEE Trans, Magn. MAG-23, 699 (1987), herein incorporated by reference, are examples of devices whose gain effect is dependent on the control of magnetic vortices in the device. The first two devices rely on the movement of Abrikosov vortices in a thin superconducting film while the third device uses the motion of Josephson vortices along the length of a tunnel junction. The long hysteretic Josephson junction vortex flow device has only been reported using low transition temperature (T.sub.c) electrode materials. However, Gerdemann et al. have reported fabricating arrays of parallel bicrystal grain boundary junctions into high T.sub.c vortex flow transistors. Single-layer vortex flow devices require a region of easily moved vortices for controlling the transport with a small external magnetic field. Vortex motion can be realized in a device by incorporating an array of weak links into the structure or by the use of a grain boundary Josephson junction.
The successful development of three-terminal high temperature superconducting devices having gain can have a significant impact on the viability of superconducting electronics. The superconducting vortex flow transistor has been demonstrated in both low T.sub.c and high temperature superconductive materials. The superconducting vortex flow transistor's speed and frequency response is dependent on the material properties and kind of vortices (Abrikosov or Josephson) responsible for vortex transport. Other superconducting flux flow and fluxonic devices include those of Hohenwater et al., Characteristics of superconducting flux-flow transistors, IEEE Trans. Magn., vol. 27, pp. 3297-3300 (March 1991), herein incorporated by reference. See also Kadin, Duality and fluxonics in superconducting devices, J. Appl. Phys., vol. 68, pp. 5741-5749 (December 1990), herein incorporated by reference. These devices are based on the motion of either Abrikosov or Josephson vortices and require a material with properties which do not substantially impede the flow of magnetic flux. The high pinning strength of YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO) has made it unsuitable for flux flow devices without modifing the YBCO in some manner such as thining or taking advantage of naturally occurring defects such as the grain boundary junction formed over a substrate step. See Martens et al., S-parameter measurements on single superconducting thin-film three-terminal devices made of high T.sub.c and low T.sub.c materials, J. Appl. Phys., vol. 65, pp. 4057-4060 (May 1989), herein incorporated by reference. See also Martens et al., Flux flow microelectronics, IEEE Trans. Appl. Super., vol. 3, pp. 2295-2302 (March 1993), herein incorporated by reference.
Researchers have sought practical, three terminal, superconducting devices for applications in hybrid technologies and on-chip integration with passive, superconducting components. Such devices included the flux-flow transistor and the fluxonic junction transistor, both of which require a superconducting material in which vortices can easily move.
High quality high temperature superconductor (HTS) thin films having "easily movable vortices" are difficult to fabricate. High quality thin films of YBCO generally have T.sub.c 's approaching 90 K (degrees Kelvin) and J.sub.c 's at 77 K greater than 1.times.10.sup.6 A/cm.sup.2 and show strong vortex pinning. In such materials, vortex motion is difficult except very close to T.sub.c or in very high magnetic fields (10's of Tesla). See Rose-Innes et al., Introduction to Superconductivity, 2nd Edition, International Series in Solid State Physics, Vol. 6, Pergamon Press, New York, at pp. 186-190 (1978), herein incorporated by reference.
Materials having easy vortex motion usually have a reduced T.sub.c and J.sub.c and are chemically unstable in the ambient environment. This is because the material within or at the grain boundaries often consists of impurities or off-stoichiometric material, causing a reduced T.sub.c, J.sub.c and chemical stability. For example, oxygen-deficient YBCO films which have reduced T.sub.c 's and J.sub.c 's as well as weak pinning have been shown to be very susceptible to damage from device processing and exposure to water-based chemicals. See L H. Allen et al., Thin film composites of Au and YBa.sub.2 Cu.sub.2 O--.sub.-.delta., Appl. Phys. Lett., vol. 66(8), pp. 1003-1005 (Feb. 20, 1995), herein incorporated by reference. Once these materials are fabricated into vortex flow devices, they degrade and change their operating characteristics with age.
Even materials that were initially high quality are susceptible to processing damage. For examnple, weak-link microbridges fabricated from high-quality materials have exhibited easy vortex motion. However, when made and used in flux flow devices, they are often operated at reduced temperatures because the T.sub.c of the microbridge is degraded by the patterning process. See Miyahara et al., Vortex Flow Characteristics of High-T.sub.c Flux Flow Transistors, J. Appl. Phys., vol 75, pp. 404 (1994), herein incorporated by reference. Furthermore, the stability with time of these devices is uncertain because of the inherent chemical instability associated with degraded superconducting material.